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LeeS

Vitrification and Glaze Fit Issues with Cone 10 Glacia

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Hi all!

I just started firing on my own after a while at the local community centre. I'm doing some slipcast cups as a first experiment using Clay Planet's dry Glacia porcelain. However, I've put two test glaze loads through the kiln and the results have been pretty disappointing.

The clay feels unvitrified. The texture is closer to bisque than finished ware, and it seems to soak up water slightly. All of the glazes I tried crazed in the bottoms of the cups. Leaving water in one of the crazed cups overnight, I can see it creeping up inside the glaze, soaking the porcelain. This seems pretty odd to me. The first firing was a solid ^10 (I used the standard guide/target/guard cones), and the work is very thin, <2 mm, so it shouldn't take long to come up to temperature. The second firing I added a soak for 30 minutes, and ^11 drooped significantly, but the clay feels the same.  A loose piece of broken bisque I set the cones on also broke very easily.

The 3 glazes I used were Tony Hansen/Digitalfire's G1947U, the transparent used at the community centre, and a modified version of that sourcing CaO from Wollastonite instead of Whiting. I've never had trouble with the community centre transparent fitting the pugged Glacia I use there, and G1947U should be even lower expansion, but all crazed. For the second firing, I tried to correct the glazes, but even super low expansion borderline unmelted glazes performed very badly.

All in all, I'm having very bad luck with the dry glacia. Here is where the questions start:

1) Has anyone seen this before, especially with this clay? Any high-level ideas for what I could be doing wrong? I don't really want to brute force it by just firing super hot/long, especially because I made it well into ^10.

2) Is it worth trying to fix the Glacia, probably by adding feldspar? I have tons of bisque, slip, and dry clay of the Glacia left, so this is somewhat attractive. It would be a bit of a pain though and I would probably have to throw out the bisque.

3) I'm kinda soured on this whole deal, so maybe it's worth just switching clays? The options are probably dry Clay Planet Pier or pugged Laguna Miller 550. I kinda want to avoid another Clay Planet formulation after the Glacia issues, but I also don't want to make slip out of the pugged Miller.

4) Maybe I should just formulate my own porcelain? On one hand, I'm pretty comfortable with the chemistry, mixing, and testing. On the other hand, it would be nice to finish some work and not just test things forever.

Thanks everyone for reading and any advice will be greatly appreciated.

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Hm, interesting thought, but I don't think that's the problem. They don't provide instructions but I deflocculate based on measured viscosity and specific gravity, not just a fixed recipe, so I have a pretty well-controlled process. Plus I bisque everything so all the casting water should be long gone. I guess maybe the particles could be too far apart or something? No idea if that's possible.

Maybe I'll mix some up some tests without deflocculant until it's just wet enough to wedge and roll out to see if that makes it better.

> Not sure, just a shot in the dark

For sure, it's a strange problem to have with a commercial clay.

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Since the clay is getting water logged after firing to cone 10 and according to Clay Planet that porcelain should have an absorption of <1% (from the description of the pugged version of that clay),  I would conclude the absorption is much higher than what is posted. There wasn't enough flux added to the mix. I would measure the absorption yourself then contact Clay Planet with your result plus the batch number and hear what they have to say. I would hold off on trying to get a glaze to fit until you have the body itself sorted out. 

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Lee:

your option 2 will work. Powder down your reclaim: you only need 500 grams. Make 4-100 grams test samples. 

Bar 1 add 5% Nep Sy. Bar 2 Add 10%  Bar 3 15% and Bar 4 20%  all four are flat tiles. Fire them flat on a kiln washed shelf to avoid plucking. Then run absorption test on each one- be sure to note on each bar the Nep Sy addition. Whatever level produces 2% or under absorption level-use that to fix the rest.  You only need to do this test once: assuming all your reclaim has had the same prior results.

From this point, weigh out dry reclaim in bulk: do not need to powder it down.  EX. 10,000 grams of dried reclaim x 10% Nep Sy = 1000 gram flux addition. From this point, slurry it down as usual. Or pug..  

Sounds like operator error at the factory!  Although I hear more and more instances of clay misbehaving. Not impossible they dug through a vein at the mine with much lower sodium content- Custer went through that in the 90's. There are some ball clays that have changed, as well as some fire clays- it happens. Clay supplier on the East coast has been having tackiness issues, drying issues. From my testing they have dug through a vein of smecite deposited in the ball clay field.

Tom

 

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I don't think it's fair that you should have to fix the clay. If you do literally have "tons of bisque, slip, and dry clay of the Glacia left" I would document your testing procedure determining absorption then contact Clay Planet. They should be the ones fixing the clay if they sold a defective product.  

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Wow I didn't expect so much helpful advice so quickly. Thanks everyone.

Quote

If you do literally have "tons of bisque, slip, and dry clay of the Glacia left" 

I didn't realize how much clay some of the production-oriented potters here go through, so I should probably clarify... It's only about 50 lbs in total of dry and slip. It's not a huge financial loss if I have to get rid of it. I also have very little reclaim that may have some plaster contamination anyway.

Quote

I would document your testing procedure determining absorption then contact Clay Planet.

I talked to Clay Planet about this. I measured ~3% +/- 0.5% absorption. They mentioned it hadn't been tested in a while, and suggested adding some feldspar. That was about it though.

I'm a bit concerned about trying to fix it because it might be a fair bit of testing work, and probably only relevant for this batch. I'm also not terribly excited about the prospect of fixing future batches, and not too thrilled with the Glacia in general. It's nice enough for throwing, but it's a bit dirty and flabby for slipcasting. I think I might just throw it in storage and switch to a new formulation.

That being said, what would everyone suggest for a new porcelain? Is it reasonable to formulate my own, or is that a world of frustration? Has anyone used either Pier or Miller 550? Is it unreasonable to avoid Pier because of this experience with the Glacia, just because it's a Clay Planet clay? No hard feelings to them, I just don't want to waste too much more time on clays that aren't performing.

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Quick update. I stopped by Clay Planet again today and they were super helpful. They're going to test out the glacia, and they hooked me up with a bag of Pier to try out. We'll see how it goes!

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